A man shot dead by police as he ate the face of another man may have been under the influence of a potent LSD-like drug called bath salts, investigators believe.

Homicide detectives looking into Saturday's gruesome incident on a Miami causeway suspect that Rudy Eugene, 31, may have overdosed before chewing on his victim's lips, ears, eyes and nose and ripping off strips of flesh with his teeth.

An eyewitness said that Eugene was "like a zombie", tearing off almost all of 65-year-old Ronald Poppo's face and growling like a wild animal at an officer who shot him.

Eugene was naked and Poppo was wearing only a shirt when police arrived, possibly a result of the delirium-inducing drug, which can have effects similar to cocaine and LSD. It can raise users' body temperature significantly and make them feel they are burning up inside.

"When a person has taken all of his clothes off and become violent, it's indicative of this excited delirium that's caused by overdose of drugs," Armando Aguilar, head of the Miami Fraternal Order of Police, told the city's WSVN news.

"What's happening is, inside their body their organs are burning up alive."Doctors at Miami's Jackson Memorial Hospital, where Poppo remained in critical condition on Tuesday, support the theory of drug-induced psychosis to explain Eugene's bizarre behaviour.

Paul Adams, an emergency room doctor, said that synthetic stimulant drugs such as bath salts, named for its powdery substance, can make users feel invincible and give them superhuman strength, but can also trigger aggression, extreme paranoia and hallucinations.

"It's the new designer drug," he told the Guardian. "It causes a state of excited delirium, raises the body temperature and causes irritability and confusion, which is heightened when combined with a lack of adequate hydration. You find yourself not making sense, and you don't control your emotions or your actions."

Bath salts, an increasingly popular drug, are responsible for rising numbers of hospital admissions and calls to the American Association of Poison Control Centers, which leapt from 304 in 2010 to more than 6,000 in 2011. A single fix for inhalation can be obtained for as low as $15, drugs experts say.

Eugene, who had a number of arrests for marijuana possession and one for assault, and Poppo, who also had an extensive history of arrests, were homeless and living on the streets close to the MacArthur causeway where a cyclist saw them fighting and called police.

An officer who arrived quickly at the scene opened fire after Eugene ignored a command to leave the other man alone.

Detective William Moreno of the Miami police department said homicide detectives were trying to piece together the events leading up to the attack. Eugene, he said, continued to chew on his victim's face even after he was shot, and it took five further shots to disable and kill him.

Security video from the adjacent Miami Herald building showed the victim writhing in agony after Eugene was dead.

Larry Vega, the cyclist who called police, gave a grisly account of the encounter and said he may seek counselling to help him get over what he saw."The guy was like tearing him to pieces with his mouth, so I told him, 'Get off!'" he told WSVN in an interview. "He just kept eating the other guy away, like ripping his skin. The police officer came over, told him several times to get off, and an officer climbed over the divider and got in front of him and said: 'Get off!'

"He told him several times and the guy just stood his head up like that with a piece of flesh in his mouth and growled. The guy, he was like a zombie, blood dripping. It was intense.

"The closest thing I've seen to it? The Walking Dead. I never thought I would see someone eating someone. It was really, really horrific."

Former classmates of Eugene, who graduated from North Miami Beach High School in 2000, were surprised to learn he was involved, remembering the former American football player as a polite man always willing to help somebody in need.

"The Rudy we know was a nice gentleman with a warm smile and funny. He's not like that at all," Victoria Forte told the Miami Herald.

Javier Ortiz, the Miami police union's vice-president, told The Guardian: "It's the most bizarre thing I've ever come across, to see the condition of this poor man who has had his face completely eaten.

"We don't know what drugs he might have been on or whether the guy was mentally ill, but there's no doubt had the officer not shot the man, the victim would have died, and there'd have been a good chance the subject would have turned on the officer."

• This article was amended on 30 May 2012. The original described bath salts as a variant of LSD. This has been corrected.